The invention relates to a power-transmission unit for variable-speed drive of machinery.
Such a power transmission unit is known from German patent document No. 34 41 877. It serves to drive machinery with variable speed under initially constant speed of the drive unit. An input shaft is connected on the one side to the drive unit and on the other side to a differential gear, either directly or by way of a load-variable hydraulic coupling. The gear output shaft which is connected to the machinery is also coupled to the differential gear. To achieve a variable output speed, an adjustable hydro-dynamic torque converter is provided, the pump wheel of which is also connected to the differential gear together with the input shaft and its turbine wheel via a brakeable shaft. For braking, the rotor of a load-degree variable brake is connected to the turbine-wheel shaft so that varying speeds can be set both at the turbine-wheel shaft (superposed shaft) and hence at the gear output. The hydrodynamic brake also serves as a freewheel unit, and it is possible without wear and in constant operation to even have the superposed shaft rotate in reverse at low speed.
In all working conditions, however, the operation of the hydro-dynamic brake is associated with torque support against the stationary housing which is, in effect, a complete conversion of the corresponding rotational energy at the turbine-wheel shaft into heat. Thereby, the power portion directed by the torque converter into the differential gear, and hence the power portion maximally converted in the brake is relatively small as the major portion of it is transmitted directly. However, it is uneconomical on a long-term basis to continuously convert a certain power portion into heat, apart from the necessity of continuously having to dispose of this heat. Particularly during high speed operation of the brake rotor, large amounts of heat must be expected.